1. Field of the Invention
The present invention relates to a method for controlling speed by controlling an output/input rotation speed ratio (hereinafter, simply referred to as a “speed ratio”) of a speed change mechanism using a hydrostatic stepless transmission (HST) (including a hydraulic mechanical transmission (HMT)) and an engine rotation speed by an electronic governor, the method being adapted to a working vehicle, such as an agricultural tractor.
2. Related Art
Conventionally, there is a well-known working vehicle, such as an agricultural tractor, comprising an HMT or another speed change mechanism using an HST; an electronic governor for controlling vehicle traveling speed; and an accelerator operation member for setting the vehicle traveling speed, such as a pedal or a lever, manipulated by an operator. The electric governor controls a speed ratio of the speed change mechanism and an engine rotation speed in correspondence to an operation degree of the accelerator operation member. International Publication WO 03/036138 discloses a tractor serving as an example of such a working vehicle.
In the tractor of the document '138, a hydraulic pump or motor of an HST of the speed change mechanism is provided with a movable swash plate whose slanting angle is controlled for changing the speed ratio of the speed change mechanism, so as to compensate the vehicle traveling speed to the reduction of the engine rotation speed, when engine load is increased or the tractor turns on a headland.
In addition, with regard to such a working vehicle (for example, an agricultural tractor), generally, a low speed range is assigned to a working (off-road) speed range, and middle and high speed ranges are assigned to an on-road traveling speed range for on-road traveling between a working area (field) and a garage.
With regard to the above-mentioned working vehicle, the vehicle traveling speed is expected to change in proportion to the operation degree of the accelerator operation member. Especially, when the speed change mechanism of the working vehicle includes an auxiliary multi-stage speed change unit combined with the HST or HMT, and the auxiliary multi-stage speed unit is set into a high speed stage for the on-road traveling, it is desired that the variation pattern of the vehicle traveling speed relative to the accelerator operation be similar to that of an ordinary passenger car. A naturally noticed governing is that, in proportion to increase of the operation degree of the accelerator operation member, the opening degree of an engine throttle valve is increased so as to increase engine rotation speed, and simultaneously, the speed ratio of the speed change mechanism is increased (that is, the speed reduction ratio is reduced).
However, if this governing is performed, the variation range of engine rotation speed becomes too wide. Especially, if the accelerator operation member is set in the high speed set range, the engine rotation speed may become larger than a rated rotation speed predetermined in consideration of the engine load. With regard to the conventional governor, the above document '138 does not disclose how the engine rotation speed and the speed ratio of the speed change mechanism are controlled in response to the operation of the accelerator operation member so as to proportionally change the vehicle traveling speed with a feeling of a passenger car.
It is important that, to prevent the engine from being overloaded, the variation range of the engine rotation speed is reduced, and the variation range of the speed change mechanism is increased compensatively. However, even when the accelerator operation member is operated rapidly, the vehicle traveling speed cannot change desirably swiftly because the slow response of the speed ratio change. Further, the reduction of variation range of the engine rotation speed is disadvantageous in ensuring sufficient engine torque against high load applied on the engine.
Further, if the governing is based on that each of the engine rotation speed and the speed ratio of the speed change unit constantly changes in direct proportion to change of the accelerator operation degree, and even if the auxiliary multi-stage speed unit is set in a low speed stage, the engine power may be excessive or insufficient at the time of starting the engine because the weight of a vehicle is not suitable, or because a vehicle is stationary on a slope. The conventional governor of the above-mentioned document '138 reduces the engine rotation speed for controlling engine load, and increases the speed ratio of the speed change mechanism so as to compensate for the reduction of engine rotation speed, thereby ensuring a desired vehicle traveling speed. However, the governing does not resolve the above problems.